The present invention relates generally to a visual image display and more particularly to a visual image display with a fiber-optic faceplate.
A visual image display with a liquid crystal layer and a fiber-optic faceplate has the advantages of reducing glare, ghost images and parallax. The advantages are due to the fibers in the faceplate controlling the optical paths of light incident to and reflecting from the liquid crystal layer. These features are very important especially in the area of pen computers. A general discussion of such a display can be found in U.S. Pat. No. 5,035,490.
An optical fiber redistributes incident light. FIG. 1 shows a ray of light incident onto one end of the fiber at an angle .OMEGA.. The light propagates through the fiber and emits from the other end as a cone of light. The cone of light has substantially uniform intensity on the directrix or ring of the cone. The angle between the axis of the fiber and the generator of the cone is approximately .OMEGA..
A faceplate has numerous fibers bundled together. All the fibers are substantially parallel to each other. FIG. 2 shows the top view of such a faceplate. Every one of those honeycomb shape structures is a fiber.
There are different types of liquid crystal layers. In one type, a liquid crystal layer is sandwiched between polarizers to form a display. A light source is positioned above one surface of the display, and a viewer looks at the display from the opposite surface. When a pre-determined electric field is applied to the liquid crystal layer, the display becomes substantially transparent. This is known as the "bright" state of the liquid crystal layer. With another pre-determined electric field, the display is significantly less transparent. This is known as the "dark" state of the liquid crystal layer. An important term for the liquid crystal layer is its contrast ratio. This ratio is defined as the ratio of the intensity of light transmitted through the layer in its "bright" state to its "dark" state. The larger the contrast ratio, the easier it is to view an image on the display.
In the "dark" state, the liquid crystal layer is not uniformly "dark". The layer appears more transparent along certain directions than along other directions. FIG. 3 shows a polar projection of the degree of darkness from the layer in its "dark" state. It is known as its conoscopic pattern.
The conoscopic pattern shows how dark the liquid crystal layer is as a function of both the azimuthal and the elevational angles from the surface of the layer. For example, the points on the line from A to C denote the degree of darkness along the X axis with an elevation angle ranging from 90.degree. to 0.degree.; point A has an azimuthal angle of 0.degree. and an elevation angle 90.degree.; point B has an azimuthal angle of 0.degree. and an elevation angle 60.degree.; and point C has an azimuthal angle of 0.degree. and elevation angle 0.degree.. Point D in the Figure has an azimuthal angle .phi. and an elevation angle .theta..
The degree of darkness can be represented by a number, the lower the number, the darker the direction. In other words, if one views the layer along a direction with a higher number, one will see more light; for example, the section with a number 5 is more transparent than that with a number 4. A general discussion on conoscopic patterns of liquid crystal layers can be found in "Conoscopic Study of Liquid Crystal After Application and Removal of the External Electric Field," written by Katsuya Miyoshi et al., and published in the Japanese Journal of Applied Physics Vol. 22, Number 12, Page 1754, 1983.
FIG. 4 shows a part of a prior art visual image display with a fiber-optic faceplate located adjacent to a liquid crystal layer. Additional layers, such as polarizers, electrodes and reflectors, are removed from the Figure for clarity. Every fiber in the faceplate is substantially perpendicular to the surface of the layer. The layer has a conoscopic pattern as shown in FIG. 3. FIG. 4 shows light propagating from region 4 in the "dark" state into the fiber. The direction of the light is approximately along an azimuthal direction of about 180.degree. and an elevation angle of about 45.degree.. FIG. 4 shows a fiber in the faceplate redistributing the light into a cone of light. The liquid crystal layer with the conoscopic pattern as shown in FIG. 3 generates numerous cones of light. Such cones of light significantly reduce the contrast ratio of the display.
It will be apparent from the foregoing that there is still a need to improve in the contrast ratio of a visual image display with a liquid crystal layer and a faceplate.